1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions and to circuits comprising such devices.
2. Introduction to the Invention
Electrical devices comprising conductive polymer compositions are well-known. Such devices comprise an element composed of a conductive polymer. The element is physically and electrically connected to at least one electrode suitable for attachment to a source of electrical power. Those factors determining the type of electrode used include the specific application, the configuration of the device, the surface to which the device is to be attached, and the nature of the conductive polymer. Among those types of electrodes which have been used are solid and stranded wires, metal foils, perforated and expanded metal sheets, and conductive inks and paints. When the conductive polymer element is in the form of a sheet or laminar element, metal foil electrodes which are directly attached to the surface of the conductive polymer, sandwiching the element, are particularly preferred. Examples of such devices are found in U.S. Pat. No. 4,426,633 (Taylor), U.S. Pat. No. 4,689,475 (Matthiesen), U.S. Pat. No. 4,800,253 (Kleiner et al), U.S. Pat. No. 4,857,880 (Au et al), U.S. Pat. No. 4,907,340 (Fang et al), and U.S. Pat. No. 4,924,074 (Fang et al), the disclosures of which are incorporated herein by reference.
As disclosed in U.S. Pat. No. 4,689,475 (Matthiesen) and U.S. Pat. No. 4,800,253 (Kleiner et al), microrough metal foils having certain characteristics give excellent results when used as electrodes in contact with conductive polymers. Thus U.S. Pat. No. 4,689,475 discloses the use of metal foils which have surface irregularities, e.g. nodules, which protrude from the surface by 0.1 to 100 microns and have at least one dimension parallel to the surface which is at most 100 microns, and U.S. Pat. No. 4,800,253 discloses the use of metal foils with a microrough surface which comprises macronodules which themselves comprise micronodules. Other documents which disclose the use of metal foils having rough surfaces, but which do not disclose the characteristics of the foils disclosed in U.S. Pat. Nos. 4,689,475 and 4,800,253, are Japanese Patent Kokai No. 62-113402 (Murata, 1987), Japanese Patent Kokoku H4-18681 (Idemitsu Kosan, 1992), and German Patent Application No. 3707494A (Nippon Mektron Ltd). The disclosure of each of these U.S., Japanese, and German documents is incorporated herein by reference.
We have found that still better results for electrodes which are in contact with a conductive polymer can be obtained by using rough-surfaces metal foils having one or both of two characteristics which are not found in the metal foils which have been used, or proposed for use, in the past. These characteristics are
(1) The protrusions from the surface of the foil should have a certain minimum average height (and preferably a certain maximum average height), as expressed by a value known as the xe2x80x9ccenter line average roughnessxe2x80x9d, whose measurement is described below. In addition, the protrusions from the surface of the foil have a certain minimum irregularity (or xe2x80x9cstructurexe2x80x9d), as expressed by a value known as the xe2x80x9creflection densityxe2x80x9d, whose measurement is also described below.
(2) The base of the foil comprises a first metal and the protrusions from the surface of the foil comprise a second metal. The first metal is selected to have high thermal and electrical conductivity, and is preferably easily manufactured at a relatively low cost. In addition, the first metal is often more likely to cause degradation of the conductive polymer than the second metal. Fracture of the protrusions, caused by thermal cycling of the device, and/or thermal diffusion of the metals at elevated temperature, exposes the second metal rather than the first metal.
Characteristic (1) is believed to be important because it ensures that the conductive polymer penetrates into the surface of the foil sufficiently to provide a good mechanical bond. However, if the height of the protrusions is too great, the polymer will not completely fill the crevices between the protrusions, leaving an air gap which will result in accelerated aging of the conductive polymer and/or more rapid corrosion of the polymer/metal interface surrounding the air gap. Characteristic (2) is based upon our discovery that thermal cycling of the device will cause fracture of some of the protrusions as a result of the different thermal expansion characteristics of the conductive polymer and the foil, so that it is important that such fracture does not expose the conductive polymer to a metal which will promote polymer degradation. In addition, it is important that a sufficient thickness of the second metal be in contact with the conductive polymer so that even if the first metal diffuses into the second metal at elevated temperature, there is little chance that the first metal will contact the conductive polymer.
In a first aspect, this invention discloses an electrical device which comprises
(A) an element composed of a conductive polymer; and
(B) at least one metal foil electrode which
(1) comprises
(a) a base layer which comprises a first metal,
(b) an intermediate metal layer which (i) is positioned between the base layer and a surface layer, and (ii) comprises a metal which is different from the first metal, and
(c) a surface layer which (i) comprises a second metal, (ii) has a center line average roughness {overscore (Ra)} of at least 1.3, and (iii) has a reflection density Rd of at least 0.60, and
(2) is positioned so that the surface layer is in direct physical contact with the conductive polymer element.
In a second aspect, this invention provides a circuit protection device which comprises
(A) an element composed of a conductive polymer which exhibits PTC behavior; and
(B) two metal foil electrodes positioned on opposite sides of the conductive polymer element, each of which electrodes comprises
(1) a base layer which comprises copper,
(2) an intermediate layer which (a) is adjacent to the base layer and (b) comprises nickel, and
(3) a surface layer which (a) comprises nickel, (b) has a center line average roughness {overscore (Ra)} of at least 1.3 and at most 2.5, (c) has a reflection density Rd of at least 0.60, and (d) is in direct physical contact with the conductive polymer element.
In a third aspect, this invention provides an electrical circuit which comprises
(A) a source of electrical power;
(B) a load; and
(C) an electrical device, e.g. a circuit protection device, of the first aspect of the invention.